Electron gun for traveling-wave tubes with a transverse magnetic field



D. CHARLES ELECTRON GUN FUR TRAVELING-WAVE TUBES WITH A TRANSVERSE MAGNETIC FIELD Filed March 18. 1950 R L... E m Q United States Patent O ELECTRON GUN FOR TRAVELlNG-WAVE TUBES WITH A TRANSVERSE MAGNETIC FIELD Daniel Charles, Paris, France, assigner to Compagnie I(lenerale de Telegraphie Sans Fil, a corporation of rance Application March 18, 1950, Serial No. 150,357

Claims priority, application France March 21, 1949 2 Claims. (Cl. 315-3) The present invention relates to travelling wave tubes, and more particularly to a novel electron gun for use in those tubes.

In United States Patent No. 2,511,407 issued June 13, 1950, a travelling-wave tube is described wherein a retardation line comprising two parallel conductors is curved to circular form and an electric field is applied between its two conductors, while a magnetic field of suitable value and direction is applied at right angles both to said electric field and to the direction of propagation of an electron beam introduced into the electron and wave interaction space between said two conductors and propagating in a circle parallel to the retardation line. In the co-pending United States patent application, Serial No. 102,896, filed on July 2, 1949, by R. Warnecke et al., a similar arrangement is applied to a system in which the retardation line and the trajectory of the electron beam are not circular but straight. In either case a cathode for generating the electron beam is placed either at the level of the negative electrode of the retardation line, or in the interaction space between the two conductors, said cathode being raised to a suitable intermediate potential. However, no arrangement is made to ensure that the electrons, once they have left the cathode and taking into account the presence of the necessary fields for the operation of the system, follow a suitable trajectory as regards position in the system in keeping with the velocity which is imparted to the electrons.

My invention relates to a solution of this problem and provides a source of electrons adapted to generate a beam which is suitable for the correct operation of the tube of the type described.

According to the invention, I provide in the tube, before the input end of the retardation line, an electron gun comprising an auxiliary focusing system of two electrodes between which an electric field is set up which is parallel to the field between the conductors of the retardation line and the value of which is equal to half that of the latter field. I position the emitting cathode of the gun in relation to this auxiliary system at such a level that the electrons penetrate into the interaction space between the conductors of the retardation line at a desired velocity, approximately equal to the phase velocity of the electromagnetic wave which is excited in the conductors of the retardation line and which is to undergo an amplification in the tube by interacting with the electron beam. Furthermore, I make the position in space of the auxiliary system with respect to the conductors of the retardation line such that the imaginary equipotential surface along which the electrons leave the auxiliary system merges into the imaginary equipotential surface that corresponds to the desired velocity of the electrons in the field between the conductors of the retardation line.

The invention will be more clearly understood by considering the non-limitative embodiment thereof illustrated in the single figure of the accompanying drawing.

This figure shows an axial section, in the plane at right angles to the lines of magnetic field, of a travelling-wave tube described in the aforesaid patent and application, and more particularly of a tube of' straight shape. It is however to be understood that everything stated in the ensuing description is also applicable to tubes of cylindrical shape, in which the distance between the conductors of the retardation line is small as compared with the radius of the cylinder, so that it may be considered that everything takes place as if the structure were fiat,

2,694,783 Patented Nov. 16, 1954 ice The tube, which is contained in an envelope 1 of insulating material, comprises the usual elements of a travelling-wave tube with a transverse magnetic field, viz. a line comprising two conductors 2 and 3, the conductor 2 for example of which is a retardation line shown schematically in the form of a helix raised to a positive potential, while the conductor 3 is a flat negative electrode. A collector 4 is provided for collecting the electron beam, and the coaxial lines 5 and 6 respectively serve for exciting the line and for collecting the amplified energy. Magnetic poles not shown, located on either side of the tube, supply a magnetic field at right angles to the plane of the drawing, a few of the lines of force of which are shown at 7. If the electric field between the conductors 2 and 3 is directed upwards in the ligure, and if the electrons are to move from left to right, the magnetic field must be directed from the observer towards the drawing. The electric field between the conductors of the line is set up by means of the source 8 that supplies the collector 4 and the retardation line 2 by means of its positive pole which is connected to ground.

By the combined action of the electric field and the transverse magnetic field, the electrons, under optimum conditions, describe in the space between the conductors 2 and 3 a trajectory at a mean velocity v substantially equal to the phase velocity of the wave in the line. ln order that said trajectory, in the absence of the radiofrequency field, may be a cycloid described along a line parallel to the retardation line, it is necessary for the electric field E and the magnetic field B to be related to each other by the equation To this velocity there corresponds in the field an equipotential level or imaginary surface 9, the potential Vo of which is determined by the kinetic energy equation:

The conditions for stability require that the electrons be introduced into the field E at the velocity v precisely along the equipotential surface 9, in order that they may follow a mean straight path at the level of th at surface.

On the other hand, the potential to be given to the retardation line is determined by the considerations of power efficiency which is substantially equal to the ratio between the difference of potentials through which the electrons pass laterally by the action of the radio-frequency field (difference of potentials between the conductor 2 and the equipotential surface 9), and the difference of potentials between the conductor 2 and the equipotential surface zero (the potential of the source of electrons being taken by convention as (l), i. e. the difference of potentials between the conductors 2 and 3 if' the conductor 3 is at the potential of the cathode. In other words, if the potential of the conductor 2 is denoted by nVo, the efliciency will be substantially equal to If a desired efficiency is given, I therefore have all the elements for determining the potential of the conductor 2, the electric field and the magnetic field to be applied to the system, and also for defining the position of the equipotential surface 9 in said system.

According to the invention, I provide before the entrance into the system of conductors 2 and 3, an auxiliary system of two beam focusing electrodes 10 and 1l which are formed for example by plates parallel to the ax1s of the system and to the magnetic field, and between which an electric field is set up (the field E being the field between the electrodes 2 and 3). The emitting cathode 12, which is heated by a filament 13 supplied from a source 14, is located for example at the level of the electrode 11 which ,in this case is connected to the potential of the cathode 12 assumed by delinition to be 0. In order to obtain the desired field, it is only necessary toconnect the electrode by means of the connection 15 to an intermediate potential obtained from the source 8 that supplies the conductor 2. The conductor 3 is connected in 'the example kdescribed to the 4electrode 11 i. enit-vis at the potential 0, without this arrangement .being limitative, since said electrodeV may also Abe raised `to acari-.ain nega tive or positive potential Vprovided that ,fit istshifted -tnansversely a suitable distance relatively to the Aeqnipoten tial surface 9.

In the arrangement shown, ,an electron that starts without initial velocity from the level of the cathode at thepotential 0, describes, in the presence of a perpendic u-lar magnetic ie1d,.a cycloid 1 6 at a velocit-ynequal to:

2E: 91am B Allherein El is the electric tieldbetween the plates 10 .and

it can be seen that if Eri? the velocity v1=v.

Owing to the fact that this velocity, which maybe represented by a line tangent to the cycloni, is parallel to the axis of the system at Vthe highestrpoint of the-'trajectory 16, it can be seen that it'said highest point is located directly at the outlet from the system 'HL-'11 sand at the inlet into the system 2-'3, I have obtained the object that my invention is intended to provide, viz. that the electrons penetrate into the spacelbetween the lconductors of the retardation line at a velocity equal to the phase `velocity of the wave to be amplified.

Furthermore, in order to obtain a smooth join without sudden variations of field, l shift the system 10-11 transversely with respect to the system 2 3 in such a manner that the equipotential surface .17, along the electrons leave the system L10- 11, is gomed tothe equipotential surface 9, the position of `willich in the system 2 3 was defined by the Equations l and 2 hereinbefore explained. Since the equipotcntial surface 9 corresponds to the potential Vu, the equipotential surface 17 corre sponds to the axis of symmetry between 1:0- :and 11, the potential to be given to the electrode 10 is 2Vo, the .distance d1 between 10 and 11 is dened by the condition anna 1 E E Since, on the other hand, the distance d between 2 and 3 is given by nya drinit can be seen that di may bemade equal to d, without this being limitativ provided that 11:4, that is to say that an eiciency of 75 is chosen.

The invention is not restricted to the example described and does not depend in particular .either on the nature of the cathode used, or on the shape of the system, or on the relative potentials and displacements which have been mentioned only in a non-limitative manner. The invention includes all the modifications which are acces sible to the expert, using the general principle which cornprises placing the emitting cathode cfa tube ofthe type described in an auxiliary focusing system in which there is an electric field which .is substantially less than the eld required ,for the operation of `theptul'ne, said'system being arranged relatively to the overall structure of the tube in snch amanner that there is a gradual io'nnng of .the two fields. As shown in the drawing, this.' latter eondition is obtained when Vthe distance R between the eqtn potential surface in the interaction space and a aratlel plane P passing through the cathode is eqna'ltcu` algebraic sum of the distance d2 between the conductor 3 and the equipotential surface of5 .same potential value in the focusing space and the distance da between the conductor 3 and the plane P.

What I claim is:

.1. Anwesen-fieldtravelling wavevtube of the type -in cluding a delay line, a base electrode parallel to said delay line and dening therewith an interaction space, means for providing in said space an electric lield and means for providing a nnegnstic field, said electric and magnetic fields being, perpendicular to each other and to the direction of said delay line and base electrode, whereby there vis provided within said interaction space an equlpotential Vplane .corresponding to the mean velocity with which electronsuwhen fed into said space, propagate 'therein -nnder -the action of said crossed lields, said tube comprising: means comprising a cathode and two electron `licensing electrodes parallel to said delay line and base electrode and defining a focusing space, means for providing in `said vfocusing space a uniform electric field l substantially less than the field in said interaction space, of same direction yand'fpar'aliel thereto, and for providing e same magnetic field therein as in said interaction space, raid cathode lbeing positioned to feed electrons into said-focusing space Yin Aa direction fperpendicelar to said focusing electrodes and parallel to the direction'oi Said electric feld,'tl1e distance between the center of said cathodeand the planetof entry of said ,electrons into said mteractlon space being substantially equal to half the QI-Ofen arch of the `cycloidal path of `said electrons uisatd licensingspace under the actionof said fields, the dtstancebetween said egiuipotental plane` and a parallel p Passing through said cathode being equal to the algebraicsnrn of thedistanne between said base electrode and the equipotennal planeofV sanne potential value in the focnsingspace and of the distance between said base electrode .and said parallel piane, and said focusing electrodesbeing posirionedrforfdelivexing said electrons into satdfinteractionspace as saon -as they leave said focusing space.

`2. In a travelling-.wave tubehaving a retardation line and: a parallel-spaced :conductor dening :an interaction space therebetween, terminal connections thereto for app'lylng apotental'differcncetherabetween, and means Vfor establishing ra: magnetic field inthe interaction space havlngdtsljnes ofiorce-direeted-.at .right angles to the electrostatic .eld and .tovt'he direction Acti-tbe line, whereby there isprovtided w'hin saidfinteraetion space an Aerynipotenttal plane oornesponding tolltemean velocity with which electrons, when into said space, propagate therein under the action of saidl crossed lelds, an electron gun comprlsingza patrol .parallel spaced electrodes delinmg a vintersting.spateY.therebetween communicating with themteraction` space, terminal connections to said electrodes for applying a potential difference therebetween, an electron emissive .cathode positioned to emit electrons into said ,focusing mace and contained in said magnetic field, whereby Athe electrons emitted by said cathode are caused. to follow a cycloidal trajectory starting from said cathode, the magnitudeof the potential difference and the distan'ccrbetween :said electrodes being such that the potentielle saidffocusingfspaceat the'tevel of thehghest peint mi said cyoloidal path. is. equal to the potential in the interaction sparat said level, said highest point being contalnedin a transverse plane .common to said spaces and delinmgortheelectromsan exit from said focusing spaand ,anfentry into the interaction space, andthe dis tance between 1said level and a parallel plane Ypassing thronghsaid `cathode beingsubstantially equal to the algebrac sum of thedistancebetween'said conductor and the corresponding equipotential and! of theY distance between allel plane.

level in said focusing space said conductor and said par- IReferences Cited inthe tile of this patent UNITED `STATES PATENTS Number .Name Date 2,232,158 Banks Feb. 18, i941 2,372,328 Labin Mar. 27, 1945 2,414,121 Pierce Jan. 14, 1947 2,511,497 Kleen et al. .lune 13, 1950 2,53 1,9.72 Doehler etal Nov. 28, 1950 2,600,509 Lerbs lune 17, 1952 2,637,904 Lei-.bs Aug. 19, 1952 

